Soil cultivation appliance, especially a hoe, operated in an electromotive manner

ABSTRACT

An electromotive soil cultivation appliance for cultivating soil in the fields of agriculture and horticulture is described, particularly a rotary hoe with an asynchronous rotary current motor for driving a rotatable cultivating tool. The soil cultivation appliance includes a frequency converter for generating a drive voltage of adjustable frequency for the rotary current motor, wherein the frequency converter is connected to a manually operated adjusting device for varying the frequency of the drive voltage. The rotary current motor provides that a nearly constant torque of the rotary current motor is maintained over a broad range of speeds of the motor that can be adjusted by means of the adjusting device. The appliance is a user-friendly soil cultivation appliance that makes it possible to achieve an efficient cultivation of the soil in lower speed ranges.

TECHNICAL SCOPE

The present invention pertains to an electromotive soil cultivationappliance for cultivating soils in the field of agriculture andhorticulture, particularly a rotary hoe with an asynchronous rotarycurrent motor for driving a rotatable cultivating tool.

Motor-driven hand-operated soil cultivation appliances are also referredto as rotary hoes or tillers and nowadays considered standard equipmentfor horticulture businesses, allotment holders and vineyards. Soilcultivation appliances of this type contain a rotary cultivating toolthat digs into and thusly cultivates the soil. Until now, the rotarydrives of the cultivating tool were primarily realized in the form ofinternal combustion engines mounted on the undercarriage of the soilcultivation appliance. Internal combustion engines, however, not onlyemit much noise, but also significant quantities of harmful substances.In addition, a high motor speed is required in order to cultivate thesoil with a sufficiently high torque.

In the meantime, hand-operated soil cultivation appliances in whichelectric motors are used for driving the cultivating tool are alsoavailable for agricultural or horticultural applications. For example,DE 42 10 816 A1 discloses a hand-operated soil cultivation appliance inwhich the drive is realized in the form of a rotary current motor. Thetool driven by the rotary current motor can be adjusted to two differenttool widths by respectively removing or reattaching outer tool segments.However, the operation with a narrower tool width requires a differenttorque than the operation with a wider tool width. Consequently, therotary current motor can be changed over between two speed ranges, inwhich different torques are generated.

One frequent problem with conventional soil cultivation appliances isthat an efficient cultivation can only be achieved if the soil has acertain consistency. In addition, it is sometimes complicated for theuser to operate the appliance. The electric motors used in soilcultivation appliance of this type also need to have a compact designand a low price.

Based on this state of the art, the present invention aims to disclose ahand-operated electromotive soil cultivation appliance for cultivatingsoils in the fields of agriculture and horticulture, particularly arotary hoe or tiller that only emits small quantities of harmfulsubstances, allows an effective and user-friendly cultivation of soilsof different consistencies and can be manufactured in a cost-efficientfashion.

DESCRIPTION OF THE INVENTION

This objective is attained with the soil cultivation appliance accordingto claim 1. Advantageous variations and additional developments of thesoil cultivation appliance form the objects of the dependent claims orcan be inferred from the following description and the embodiments.

The hand-operated electromotive soil cultivation appliance according tothe invention comprises an asynchronous rotary current motor for drivinga rotatable cultivating tool. The soil cultivation appliance comprises afrequency converter for generating a drive voltage of adjustablefrequency for the rotary current motor, wherein the frequency converteris connected to a manually operated adjusting device for varying thefrequency of the drive voltage. The rotary current motor is realized insuch a way, preferably due to a suitably chosen number of poles andturns of the stator, that a nearly constant torque of the rotary currentmotor is maintained over a broad range of motor speeds that can beadjusted by means of the adjusting device. The broad speed rangepreferably covers speeds between 20 and more than 2000 rpm, particularlyup to approximately 6500 rpm.

An even more advantageous speed range of the rotary current motor coversspeeds between 10 and more than 3000 rpm.

An approximately constant torque over a broad speed range can be ensuredby suitably adapting the number of poles on one hand and the number ofturns of the stator of the rotary current motor on the other hand.

Commercially available low-cost rotary current motors are designed for acertain speed, i.e., their torque decreases significantly above andbelow this speed. The torque in the lower speed range can be raised byincreasing the number of poles of conventional rotary current motors.The torque in the upper speed range can be increased by reducing thenumber of turns and thusly lowering the inductive resistance in thisupper speed range. Another option for raising the torque in the lowerspeed range consists of increasing the number of turn grooves on thestator, i.e., utilizing a stator with a larger number of grooves. One ofthe aforementioned measures or a combination of the aforementionedmeasures make(s) it possible to realize conventional low-cost andlight-weight rotary current motors such that they are suitable for useas rotary current motors in the described soil cultivation appliance.

The option of varying the frequency generated by the frequency converterand therefore the speed of the rotary current motor that preferablyconsists of a three-phase rotary current motor makes it possible to alsovariably utilize the described soil cultivation appliance with a nearlyconstant torque at comparatively low speeds. The inventor recognizedthat a very effective soil cultivation is frequently achieved at lowerspeeds, particularly when cultivating hard or stony soils. The option ofregulating the speed over a broad range without causing a significantdecrease of the torque makes it possible to effectively cultivate soilsin dependence on their consistency.

The speed is preferably adjusted or varied with the aid of apotentiometer that is arranged at an easily accessible location, forexample, on the handlebar grip or the handlebar of the soil cultivationappliance. This enables the user to easily vary the motor speed, forexample, by turning the handlebar grip.

The described soil cultivation appliance preferably comprises anelectromechanical control for reversing the polarity of the rotarycurrent motor and thusly reversing the rotating direction. The polarityreversal is triggered with a switching element that is preferablymounted on the handlebar grip of the appliance. The electromechanicalcontrol also comprises a speed limiter for limiting the speed of therotary current motor in the reverse mode. This speed limiter limits themaximum speed in the reverse mode to less than 50% of the maximum speedsattainable in the forward mode.

According to one additional development of the described appliance, theelectromechanical control is connected to another switching element andrealized such that the rotary current motor is only supplied with powerif the user continuously holds the additional switching element in thedepressed position against the force of a spring. The power supply isinterrupted when the user releases this switching element. Thisadditional switching element in the form of a so-called dead man'sswitch is preferably arranged on the handlebar grip or the handlebar andassists in preventing accidents. The electromechanical control is alsorealized in such a way that both switching elements need to be actuatedsimultaneously in order to operate the appliance. This prevents theappliance from being unintentionally started. In addition, an electricinterlocking system is preferably provided between the two switchingelements in order to ensure that the appliance can only be changed overbetween the forward mode and the reverse mode when the electricaloperation of the appliance is interrupted.

The adjusting device for adjusting the speed preferably forms part ofthe electromechanical control. The adjusting device can be electricallyintegrated into the control circuit if it is realized in the form of apotentiometer. The control comprises a microprocessor in one embodimentof the invention. This means that an adjusting device in the form of apotentiometer is not required in this case. The speed is adjusted, forexample, with the switching element for changing over between theforward mode and the reverse mode, wherein the speed can be increased ordecreased by depressing this switching element for a certain period oftime. The variation of the frequency or the speed, respectively, isachieved by feeding a 0-10 V, 4-20 mA signal to the corresponding inputof the frequency converter.

A very user-friendly operation of the soil cultivation appliance can beachieved with the latter embodiments of the soil cultivation appliance,particularly those with reversible rotating direction, because the useris able to easily release a stuck soil cultivation appliance byswitching into the reverse mode.

In another advantageous embodiment, the frequency converter is realizedin such a way that it not only delivers the drive voltage for the rotarycurrent motor, but also a direct voltage for operating theelectromechanical control. This makes it possible to additionally reducethe weight of the appliance because a separate power transformer is nolonger required.

Another advantage of the described soil cultivation appliance can beseen in that no additional gears are required. The soil cultivationappliance can be operated in the forward as well as in the reverse modeby directly connecting the cultivating tool to the rotary current motorvia a worm gear.

BRIEF DESCRIPTION OF THE FIGURES

The soil cultivation appliance according to the invention is brieflydescribed once again below with reference to one embodiment that isillustrated in the figures. The figures show:

FIG. 1, a highly schematic representation of a soil cultivationappliance according to the present invention;

FIG. 2, a schematic representation of the electrical components of thesoil cultivation appliance according to the invention;

FIG. 3, an exemplary circuit for realizing the electromechanical controlof the soil cultivation appliance according to the invention, and

FIG. 4, an exemplary torque curve of the rotary current motor of thesoil cultivation appliance according to the invention.

WAYS FOR REALIZING THE INVENTION

FIG. 1 shows a highly schematic representation of one typical design ofa soil cultivation appliance that may also be used for the soilcultivation appliance according to the present invention. The soilcultivation appliance usually comprises a frame 1 that may be providedwith a fold-down wheel 2 for transporting the appliance. A hoeing ortilling tool 3 is rotatively suspended on the frame 1 such that it digsinto the soil. The tool 3 is driven by an asynchronous rotary currentmotor 4 that is connected to the tool 3, for example, via a worm gear. Asteering arm 5 with a handlebar grip or handlebar 6 is mounted on theframe 1 and enables the user to steer the soil cultivation appliance.Two switching elements 7, 8 as well as a potentiometer 9 are arranged onthe handlebar grip 6. Electric lines are provided for connecting theswitching elements 7, 8 and the potentiometer 9 to an electromechanicalcontrol of the rotary current motor 4 that is not illustrated in thisfigure.

In order to distinguish between an intentional and an unintentionalstart of the appliance, two independent electromechanical switchingfunctions need to be activated in this particular soil cultivationappliance. The switching element 7 in the form of a prestressed handleserves as an emergency stop. This switching element 7 needs to beconstantly held in the activated position by the user during theoperation of the soil cultivation appliance. Once this switching element7 is released, it immediately assumes its idle position under theinfluence of a spring force such that the power supply between thefrequency converter and the rotary current motor 4 is interrupted. Thesecond switching element 8 is realized in the form of a pushbutton witha neutral position (center position with no switching function) as wellas a forward and reverse position in the embodiment shown. This makes itpossible to select the rotating direction of the rotary current motorand therefore the driving direction of the soil cultivation appliance.

The appliance is started by actuating or depressing the first switchingelement 7 and setting the switching element 8 for selecting the drivingdirection accordingly. The motor starts as soon as a driving directionis selected. The motor speed or driving speed can be varied with the aidof the easily accessible potentiometer 9. Alternatively, it would alsobe possible to utilize an electronic microprocessor in order tointegrate the speed adjusting function into the switching element forselecting the forward or reverse mode, i.e., the speed is increased ordecreased based on the time during which the switching element isdepressed in the forward position or the reverse position.

The electromechanical control limits the maximum speed or driving speedwhen the reverse mode is selected. This minimizes the risk of accidentsdue to the difficult handling of the machine. An additional safety inthe form of an electrical interlocking system is also installed betweenthe switching element 7 for the emergency stop and the switching element8 for selecting the forward or reverse mode. This ensures that thedriving direction can only be changed after the power supply between thefrequency converter and the rotary current motor of the appliance isinterrupted.

FIG. 2 schematically shows the essential electrical components of thesoil cultivation appliance according to the invention. This figure showsthe motor 4 that is supplied with a drive voltage by the frequencyconverter 10. The potentiometer 9 makes it possible to variably adjustthe frequency of the drive voltage over a broad frequency range thatlies between approximately 5 Hz and 120 Hz in the embodiment shown. Thepotentiometer 9 forms part of the electromechanical control 11 thatserves for controlling the operating mode (forward-reverse) of the motor4 with the aid of the corresponding switching elements 7, 8 shown inFIG. 1.

The motor used in the described embodiment consists of a commerciallyavailable three-phase rotary current motor with four poles that operateswith an alternating voltage of 220 V, wherein said motor is adapted insuch a way that a nearly constant torque is achieved, i.e., a torquethat the varies by less than 10% over the entire speed range of themotor that covers speeds between 10 and 3600 rpm and can be adjustedwith the potentiometer 9. For this purpose, the number of turns of theconventional rotary current motor was reduced from 62 turns to 42 turnsin order to also attain a sufficiently low inductive resistance forgenerating the required torque at the maximum operating frequency of 120Hz. The adaptation of the conventional rotary current motor alsoresulted in a power increase from 1.1 to 1.7 kW. This demonstrates thatthe adaptation according to the invention makes it possible to maintainthe torque approximately constant over a broad speed range and togenerate the same power with a smaller and therefore lighter motor.

A rotary current limiter is used for limiting the frequency of the drivevoltage for this motor to a maximum frequency of 35 Hz in the reversemode.

FIG. 3 shows an example for realizing the circuit of theelectromechanical control used in the soil cultivation applianceaccording to the invention. Among other things, this circuit comprisesthe reverse mode speed limiter 12, the potentiometer 9, the emergencystop switching element 7 and the two switching functions of theswitching element 8.

FIG. 4 shows the torque curve of another motor used for operating thesoil cultivation appliance according to the invention. In thisembodiment, a commercially available 0.37 kW rotary current motor thathas two poles and operates on 220 V alternating voltage is adapted insuch a way that it has a nearly constant torque over a speed rangebetween 20 and 6800 rpm that can be adjusted with the aid of thepotentiometer 9. For this purpose, a laminated core with 24 grooves isused in the motor instead of the originally provided laminated core with18 grooves, and the winding is adapted accordingly. This measuresurprisingly resulted in a practically constant torque over the broadspeed range as illustrated in the figure.

LIST OF REFERENCE SYMBOLS

-   1 Frame-   2 Fold-down wheel-   3 Cultivating tool-   4 Rotary current motor-   5 Steering arm-   6 Handlebar grip or handlebar-   7 First switching element-   8 Second switching element-   9 Potentiometer-   10 Frequency converter-   11 Electromechanical control-   12 Speed limiter

1-14. (canceled)
 15. An electromotive soil cultivation appliance forcultivating soil comprising a rotary hoe with an asynchronous rotarycurrent motor for driving a rotatable cultivating tool, a frequencyconverter for generating a drive voltage of adjustable frequency for therotary current motor, wherein the frequency converter is connected to amanually operated adjusting device for varying frequency of the drivevoltage, and wherein the rotary current motor is constructed andarranged so that a nearly constant torque of the rotary current motor ismaintained over a range of speeds of the motor and are adjustable by anadjusting device, wherein the nearly constant torque is maintained byadapting or selecting a number of poles and number of turns of therotary current motor, and wherein a sufficiently high torque ismaintained in a lower speed range by providing a correspondingly highnumber of turn grooves and/or poles.
 16. The soil cultivation applianceaccording to claim 15, wherein the rotary current motor maintains saidnearly constant torque over a speed range of the rotary current motorfrom 20 to 6500 rpm.
 17. The soil cultivation appliance according toclaim 15, wherein the rotary current motor maintains said nearlyconstant torque over a speed range of the rotary current motor from 10to more than 3000 rpm.
 18. The soil cultivation appliance according toclaim 15, further comprising maintenance of a sufficiently low inductiveresistance in a higher speed range by selecting a correspondingly smallnumber of turns.
 19. The soil cultivation appliance according to claim15, wherein the torque of the rotary current motor varies by no morethan 10% over a range of motor speeds.
 20. The soil cultivationappliance according to claim 15, wherein the frequency converter and theadjusting device generate a drive voltage for the rotary current motorthat has a maximum frequency in excess of 100 Hz.
 21. The soilcultivation appliance according to claim 15, further comprising anelectromechanical control for reversing polarity of the rotary currentmotor such that the soil cultivation appliance is operable in a forwardmode or a reverse mode, wherein a speed limiter limits speed of therotary current motor in the reverse mode to no more than 50% of a speedattainable in the forward mode.
 22. The soil cultivation applianceaccording to claim 21, wherein the electromechanical control isconnected to a first switching element and a second switching elementthat are actuated simultaneously in order to start the appliance,wherein the first switching element must be continuously held in adepressed position by a user against a force of a spring in order tooperate the appliance and the second switching element provides forselection of the forward mode or the reverse mode.
 23. The soilcultivation appliance according to claim 22, wherein theelectromechanical control interrupts electrical operation of theappliance for changing over between the forward mode and the reversemode.
 24. The soil cultivation appliance according to claim 21, whereinthe frequency converter has a direct voltage output for supplying theelectromechanical control with power.
 25. The soil cultivation applianceaccording to claim 21, wherein the electromechanical control comprises amicroprocessor control.
 26. The soil cultivation appliance according toclaim 15, wherein the adjusting device comprises a potentiometer.